Selective reabsorption is the process whereby certain molecules (e.g.

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, usi ...

and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

s), after being filtered out of the capillaries along with

nitrogenous waste Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, w ...

products (i.e.

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...

) and water in the

glomerulus ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...

, are reabsorbed from the filtrate as they pass through the

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ...

. Selective reabsorbtion occurs in the PCT (proximal convoluted tubule). The PCT is highly permeable meaning it is easy for molecules to diffuse through it.

A basic outline of the process

*The

co-transport In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellul ...

sodium-potassium pump actively transports sodium out of the PCT (

proximal convoluted tubule The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. It can be further classified into the proximal convoluted tubule (PCT) and the proximal straig ...

) wall (using energy from converting

ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals, men's professional tennis governing body * American Technical Publishers, employee-owned publishing company * ', a Danish pension * Armenia Tree Project, non ...

ADP Adp or ADP may refer to: Aviation * Aéroports de Paris, airport authority for the Parisian region in France * Aeropuertos del Perú, airport operator for airports in northern Peru * SLAF Anuradhapura, an airport in Sri Lanka * Ampara Air ...

+ Pi) to maintain a low Na⁺ concentration gradient in the wall. *This low

concentration gradient Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) o ...

means that Na⁺ ions from the

filtrate can easily passively diffuse into the wall of the PCT. *However, the Na⁺ ions cannot diffuse freely across the membrane, but can only enter through special

transporter Transporter may refer to: * Transporter (vehicles), types of vehicles designed to transport items * Transporter wagon, a railway car designed to carry another railway car * Volkswagen Transporter, a model of van * Transporter bridge, a bridge wh ...

(carrier) proteins in the membrane of the wall. *There are several different kinds of these transporter proteins, each of which transports another molecule, such as

s. The concentration gradient for the sodium provides the energy to pull in these other molecules into the wall of the PCT. *As the substances listed above (Na⁺ ions, amino acids and glucose) enter the wall of the PCT, so does 65–70% of the water in the glomerulus

filtrate Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...

via

osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...

. Water can move freely through the wall of the PCT (it does not require a transporter protein). Nearly all the rest of the water is reabsorbed into the blood in the

loop of Henle In the kidney, the loop of Henle () (or Henle's loop, Henle loop, nephron loop or its Latin counterpart ''ansa nephroni'') is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after i ...

and the

collecting duct system The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct system is the last part of nephron and participates in elect ...

. *However, as

is a small molecule it can pass easily through the membrane of the PCT wall. As the concentration of urea in the filtrate is significantly higher than in the blood, around 50% of urea on the filtrate is reabsorbed.

References

* {{cite book , title=Biology 2 , author1=Mary Jones , author2=Jennifer Gregory , author3=Yudhis McMarty , author4=Noel Jackson. , publisher=Cambridge University Press, year=2004 *Mr A.J.Yorath, M.C.S. *Kit Briant & Andrew Gove. Renal physiology